The present invention relates to a development device used for an electrophotography type image forming device, such as a photocopier, a printer, and a facsimile device; and a charging method used in the development device; and an image forming device having the development device.
An electrophotography type image forming device (electrophotography device), such as a photocopier, a printer, or a facsimile device generally includes a LSU (Laser beam Scanner Unit), a photoconductive drum, and a development device. Here, the LSU irradiates a rotating photoconductive drum with a laser-beam so as to form an electrostatic latent image on a surface of the photoconductive drum. Then, the development device further supplies toner to the photoconductive drum so as to develop (visualize) the electrostatic latent image.
Further, the development device includes a development roller provided to be adjacent and opposite to the photoconductive drum. The development device uses, for example, a feed roller (toner feeding roller) for supplying toner on a surface of the development roller, and rotates the development roller, so as to sequentially supply the toner to all of the electrostatic latent images on the photoconductive drum.
Incidentally, in such a development device, the electrostatic latent image on the photoconductive drum absorbs the toner due to electrostatic force so as to carry out development. Therefore, it is required to charge the toner by some methods.
For example, in case of a development device using nonmagnetic toner of one-component system for development of an electrostatic latent image, the toner is sequentially supplied to the surface of a development roller by a feed roller in the circumferential direction, while the toner is held and carried by rotation of the development roller. Also, the thickness of the toner is controlled by a thickness control blade, which is provided downstream of the feed roller in the rotation direction of the development roller, while the toner is charged due to friction with the thickness control blade (friction charging).
The toner is held in this state until carried to an opposite portion to a photoconductor. The portion is provided further downstream in the rotation direction of the development roller. Then, the toner is supplied to an electrostatic latent image on the surface of the photoconductor due to electrostatic force. As a result, the electrostatic latent image is developed (visualized) as a toner image.
Note that, the developer used in the development device may be a one-component type magnetic toner containing magnetic powder, or a two-component type developer in which the toner is mixed with a carrier.
In this manner, the toner is charged by friction with the blade while its thickness is controlled by the blade applied to the development roller with a great pressure (F).
Considering this arrangement in terms of the energy budget, driving energy (Ek) supplied to the development roller is converted to toner thickness control energy (Es) and toner charging energy (Et). Also, the driving energy is partly consumed as heat loss energy (E1).
Namely, the following equation (1) is satisfied as the basic equation of the energy budget in such a friction charging method.
Ek=Es+Et+E1xe2x80x83xe2x80x83(1) 
The heat loss energy (E1) generated by such a friction charging brings about such as destruction of toner, fusion of softened toner into the surface of the blade. This further causes degradation of friction charging property between the toner and the surface of the blade.
Further, in recent years, as one of energy saving technologies, melioration of toner has been in progress. For example, the softening point of toner is decreased for reduction of fixing energy, also the number of pigments of toner is increased for an improvement of coloring property.
However, as described, the foregoing friction charging method performing control of the thickness of the toner and the charging of the toner at the same time is carried out with a great thermal load, and therefore the foregoing improved toner cannot be used for the method. In this regard, a charging method with a small thermal load has been required in order to cope with the improved toner.
Further, the relation between the heat loss energy (E1) and the applied pressure (F) satisfies the following equation (2) where C1 is a proportional constant.
heat loss energy (E1)=C1xc3x97pressure (F)xe2x80x83xe2x80x83(2) 
According to this equation (2), it is effective to decrease the applied pressure for reducing the thermal load with respect to the toner.
Further, the driving energy (Ek) of the development roller satisfies the following equation (3) where C2 is a proportional constant.
driving energy (Ek)=C2xc3x97pressure (F)xe2x80x83xe2x80x83(3) 
According to the foregoing equations (1) through (3), the applied pressure (F) satisfies the following equations (4).
C2xc3x97F=Es+Et+C1xc3x97F 
(C2xe2x88x92C1)xc3x97F=Es+Etxe2x80x83xe2x80x83(4) 
The equations revealed that, in order to realize pressure reduction, it is effective to use the method in which the toner charging energy (Et) does not rely on the applied pressure (F).
Then, the blade of friction charging method is here set to separately performs the toner thickness control function and the toner charging function. More specifically, the applied pressure (F) is mainly used as the toner thickness control energy (Es), while the toner charging energy (Et) relies on light energy instead of the pressure (F).
In such a case, the thermal load can be reduced and therefore, it is possible to prevent destruction of the toner, i.e., degradation of the toner, or fusion of the toner into the blade. Thus, reliability of the development operation can be increased. Further, this charging method may also be used with the improved toner which is for realizing reduction of fixing energy or improvement of coloring ability.
Further, as one example of the charging method using light energy, Japanese Laid-Open Patent Application Tokukaihei 07-281473/1995 (published on Oct. 27, 1995) discloses a toner charging method which controls the toner charging quantity by irradiating the toner with light in a developer tank after including a photochromic compound in the toner.
However, in such a charging method using light energy, assuming that all of the members in the development device are turned on, the toner is not thoroughly charged at the time that the light irradiation has just been started, i.e., the toner carriage has just been started.
In this case, the toner, which is still in an uncharged state, is carried to a latent image holding body by a carriage section. This causes adherence of the uncharged toner to the electrostatic latent image or scatter of the toner inside of the device. As a result, some blank spots appear on the image, or the image becomes unfocused, thus decreasing printing quality.
Further, in the configuration of the Japanese Laid-Open Patent Application Tokukaihei 07-281473/1995, which uses light energy, the irradiation of the toner is carried out in the developer tank, thus failing to ensure stability of the toner charging.
Further, even when the toner on the toner carriage section (developer carriage section) is irradiated outside of the developer tank, i.e., when electrons are applied to the toner by using photoelectric effect of a photoelectron emitting section, the toner may not be sufficiently charged at the time that the toner carriage has just been started, and therefore the uncharged toner adheres to the surface of the photoelectron emitting section, thereby decreasing toner charging property of the surface of the photoelectron emitting section.
The present invention is made in view of the foregoing conventional problems, and an object is to provide a development device with a small thermal load with respect to the developer, and capable of stably charging the developer even immediately after the beginning of the carriage of developer by a developer carriage section, and also a charging method used in the development device, and an image forming device including the development device.
In order to solve the foregoing problems, a development device according to the present invention includes: a charging section having an electron emission section for applying electrons generated in the electron emission section due to a photoelectric effect to a developer so as to charge the developer; an irradiation section for irradiating the electron emission section with light so as to cause the electron emission section to generate the electrons; a developer carriage section for holding the developer thus charged, and performing carriage of the developer to a latent image holding body having an electrostatic latent image on a surface; and a control section for performing control of (a) application of a voltage to the irradiation section, (b) application of a voltage to the electron emission section, (c) application of a voltage to the developer carriage section, and (d) beginning of the carriage of the developer by the developer carriage section, at predetermined timings, respectively, the electrostatic latent image on the latent image holding body being developed by the developer which has been charged.
Generally, in a charging method using photoelectric effect, when all members of the development device (at least including the irradiation section, the electron emission section, and the developer carriage section) are turned on at the same time, the developer cannot be sufficiently charged immediately after the irradiation from the irradiation section, i.e., immediately after the beginning of the carriage of developer by the developer carriage section. Namely, the carriage of developer is started immediately after the image forming request even though the electrons in the electron emission section have not been sufficiently induced. As a result, the uncharged developer is carried by the developer carriage section.
As with the foregoing example, in the presence of the uncharged developer on the developer carriage section, the developer may be charged apparently in a reverse polarity due to contact and friction between the developer in the process of the carriage. More specifically, proper charging may be interfered by the peripheral developer. The developer charged in a reverse polarity gravitates toward the electron emission section and further adheres to the electron emission section. When the developer adheres to the electron emission section due to such a phenomenon, the adhering developer interferes the emission of the electrons in the electron emission section, thus decreasing developer charging ability in the electron emission section. As a result, charging of the carried developer becomes unstable.
Further, if the developer, which is still in an uncharged state, is carried to the latent image holding body by the developer carriage section, there arises adherence of the uncharged developer to the electrostatic latent image or scatter of the developer inside of the image forming device including the development device. As a result, blank spots appear on the image, or the image becomes unfocused, thereby decreasing printing quality.
However, according to the foregoing configuration, the control section controls the application of a voltage to the irradiation section, the application of a voltage to the electron emission section, the application of a voltage to the developer carriage section, and the beginning of carriage of the developer by the developer carriage section, at predetermined timings, thus stably charging the developer to be a desirable quantity even immediately after the beginning of the carriage of developer.
As a result, it is possible to prevent the decrease of charging ability in the electron emission section, thus stabilizing the charging of the carried developer.
Further, for example, the developer will not scatter inside of the image forming device having the development device, thus preventing the decrease of printing quality due to the blank spots on the image or the unfocused printing.
Since the developer is charged by using photoelectric effect, the thermal load with respect to the developer can be reduced. Therefore, it is possible to prevent destruction of the developer, or fusion of the developer into the charging section. Further, the development device can accept the use of improved developer which is made for realizing reduction of fixing energy or improvement of coloring ability.
In order to solve the foregoing problems, a charging method according to the present invention includes the step of: carrying out image forming by supplying the developer, which has been charged, to a latent image holding body having the electrostatic latent image on a surface, and transferring the developer image from the latent image holding body to a recording medium, wherein: (a) application of a voltage to an irradiation section which performs light irradiation, (b) application of a voltage to an electron emission section which induces electrons thereof by a photoelectric effect with the light irradiation, so as to discharge the electrons, (c) application of a voltage to a developer carriage section which performs carriage of the developer to the latent image holding body having the electrostatic latent image on the surface, and (d) beginning of the carriage of the developer by the developer carriage section, are carried out at predetermined timings, respectively.
According to the foregoing method, the application of a voltage to the irradiation section, the application of a voltage to the electron emission section, the application of a voltage to the developer carriage section, and the beginning of carriage of the developer by the developer carriage section, are controlled at predetermined timings, thus stably charging the developer to be a desirable quantity even immediately after the beginning of the carriage of developer, thus stabilizing the charging of the carried developer.
Further, for example, when using the foregoing charging method in an image forming device, the developer will not scatter inside of the image forming device, thus preventing the decrease of printing quality due to the blank spots on the image or the unfocused printing.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.